How to ignore user s time zone and force Date use specific time zone

Question

In an JS app  I receive timestamp  eq  1270544790922  from server  Ajax    Basing on that timestamp I create Date object using   var  date   new Date     date setTime 1270544790922     Now   date decoded timestamp in current user locale time zone  I don t want that   I would like  date to convert this timestamp to current time in city of Helsinki in Europe  disregarding current time zone of the user     How can I do that

User · Answer

Presuming you get the timestamp in Helsinki time, I would create a date object set to midnight January 1 1970 UTC (for disregarding the local timezone settings of the browser). Then just add the needed number of milliseconds to it.

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var _date = new Date( Date.UTC(1970, 0, 1, 0, 0, 0, 0) );_x000D_
_date.setUTCMilliseconds(1270544790922);_x000D_
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alert(_date); //date shown shifted corresponding to local time settings_x000D_
alert(_date.getUTCFullYear());    //the UTC year value_x000D_
alert(_date.getUTCMonth());       //the UTC month value_x000D_
alert(_date.getUTCDate());        //the UTC day of month value_x000D_
alert(_date.getUTCHours());       //the UTC hour value_x000D_
alert(_date.getUTCMinutes());     //the UTC minutes value

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Watch out later, to always ask UTC values from the date object. This way users will see the same date values regardless of local settings. Otherwise date values will be shifted corresponding to local time settings.

User · Answer

I have a suspicion  that the Answer doesn t give the correct result  In the question the asker wants to convert timestamp from server to current time in Hellsinki disregarding current time zone of the user   It s the fact that the user s timezone can be what ever so we cannot trust to it   If eg  timestamp is 1270544790922 and we have a function   var  date   new Date     date setTime 1270544790922   var  helsenkiOffset   2 60 60   maybe 3 var  userOffset    date getTimezoneOffset   60 60   var  helsenkiTime   new Date  date getTime    helsenkiOffset  userOffset     When a New Yorker visits the page  alert  helsenkiTime  prints   Tue Apr 06 2010 05 21 02 GMT-0400  EDT    And when a Finlander visits the page  alert  helsenkiTime  prints   Tue Apr 06 2010 11 55 50 GMT 0300  EEST    So the function is correct only if the page visitor has the target timezone  Europe Helsinki  in his computer  but fails in nearly every other part of the world  And because the server timestamp is usually UNIX timestamp  which is by definition in UTC  the number of seconds since the Unix Epoch  January 1 1970 00 00 00 GMT   we cannot determine DST or non-DST from timestamp   So the solution is to DISREGARD the current time zone of the user and implement some way to calculate UTC offset whether the date is in DST or not  Javascript has not native method to determine DST transition history of other timezone than the current timezone of user  We can achieve this most simply using server side script  because we have easy access to server s timezone database with the whole transition history of all timezones   But if you have no access to the server s  or any other server s  timezone database AND the timestamp is in UTC  you can get the similar functionality by hard coding the DST rules in Javascript   To cover dates in years 1998 - 2099 in Europe Helsinki you can use the following function  jsfiddled     function timestampToHellsinki server timestamp        function pad num            num   num toString            if  num length    1  return  0    num          return num             var  date   new Date         date setTime server timestamp        var  year    date getUTCFullYear            Return false  if DST rules have been different than nowadays      if   year lt  1998  amp  amp   year gt 2099  return false          Calculate DST start day  it is the last sunday of March     var start day    31 -     5    year    4    4    7        var SUMMER start   new Date Date UTC  year  2  start day  1  0  0            Calculate DST end day  it is the last sunday of October     var end day    31 -     5    year    4    1    7       var SUMMER end   new Date Date UTC  year  9  end day  1  0  0            Check if the time is between SUMMER start and SUMMER end        If the time is in summer  the offset is 2 hours        else offset is 3 hours     var hellsinkiOffset   2   60   60   1000      if   date  gt  SUMMER start  amp  amp   date  lt  SUMMER end  hellsinkiOffset        3   60   60   1000          Add server timestamp to midnight January 1  1970        Add Hellsinki offset to that      date setTime server timestamp   hellsinkiOffset       var hellsinkiTime   pad  date getUTCDate                 pad  date getUTCMonth             date getUTCFullYear                pad  date getUTCHours                pad  date getUTCMinutes            pad  date getUTCSeconds          return hellsinkiTime      Examples of usage   var server timestamp   1270544790922  document getElementById  time   innerHTML    The timestamp      server timestamp     is in Hellsinki      timestampToHellsinki server timestamp    server timestamp   1349841923   1000  document getElementById  time   innerHTML      lt br gt  lt br gt The timestamp      server timestamp     is in Hellsinki     timestampToHellsinki server timestamp    var now   new Date    server timestamp   now getTime    document getElementById  time   innerHTML      lt br gt  lt br gt The timestamp is now     server timestamp     and the current local time in Hellsinki is     timestampToHellsinki server timestamp      And this print the following regardless of user timezone   The timestamp 1270544790922 is in Hellsinki 06 03 2010 12 06 30  The timestamp 1349841923000 is in Hellsinki 10 09 2012 07 05 23  The timestamp is now 1349853751034 and the current local time in Hellsinki is 10 09 2012 10 22 31   Of course if you can return timestamp in a form that the offset  DST or non-DST one  is already added to timestamp on server  you don t have to calculate it clientside and you can simplify the function a lot  BUT remember to NOT use timezoneOffset    because then you have to deal with user timezone and this is not the wanted behaviour

User · Answer

You could use setUTCMilliseconds    var  date   new Date     date setUTCMilliseconds 1270544790922

User · Answer

Just another approach   x000D   x000D  function parseTimestamp timestampStr    x000D    return new Date new Date timestampStr  getTime      new Date timestampStr  getTimezoneOffset     60   1000    x000D     x000D   x000D    Sun Jan 01 2017 12 00 00 x000D  var timestamp   1483272000000  x000D  date   parseTimestamp timestamp   x000D  document write date   x000D   x000D   x000D    Cheers

User · Answer

To account for milliseconds and the user s time zone  use the following   var  userOffset    date getTimezoneOffset   60 1000     user s offset time var  centralOffset   6 60 60 1000     6 for central time - use whatever you need  date   new Date  date getTime   -  userOffset    centralOffset      redefine variable

User · Answer

Use this and always use UTC functions afterwards e g  mydate getUTCHours     x000D   x000D     function getDateUTC str            function getUTCDate myDateStr               if myDateStr length  lt   10                     const date   new Date myDateStr     is already assuming UTC  smart - but for browser compatibility we will add time string none the less                 const date   new Date myDateStr trim      T00 00 00Z                    return date               else                  throw  only date strings  not date time                                    function getUTCDatetime myDateStr               if myDateStr length  lt   10                   throw  only date TIME strings  not date only                else                  return new Date myDateStr trim     Z      this assumes no time zone is part of the date string  Z indicates UTC time zone                                            let rv                        if str  amp  amp  str length               if str length  lt   10                   rv   getUTCDate str                else if str length  gt  10                   rv   getUTCDatetime str                           else              rv                         return rv         console info getDateUTC  2020-02-02   toUTCString      var mydateee2   getDateUTC  2020-02-02 02 02 02    console info mydateee2 toUTCString         you are free to use all UTC functions on date e g  console info mydateee2 getUTCHours    console info  all is good now if you use UTC functions   x000D   x000D   x000D

User · Answer

A Date object s underlying value is actually in UTC  To prove this  notice that if you type new Date 0  you ll see something like  Wed Dec 31 1969 16 00 00 GMT-0800  PST   0 is treated as 0 in GMT  but  toString   method shows the local time   Big note  UTC stands for Universal time code  The current time right now in 2 different places is the same UTC  but the output can be formatted differently   What we need here is some formatting  var  date   new Date 1270544790922       outputs  gt   Tue Apr 06 2010 02 06 30 GMT-0700  PDT    for me  date toLocaleString  fi-FI     timeZone   Europe Helsinki         outputs  gt   6 4 2010 klo 12 06 30   date toLocaleString  en-US     timeZone   Europe Helsinki         outputs  gt   4 6 2010  12 06 30 PM    This works but     you can t really use any of the other date methods for your purposes since they describe the user s timezone  What you want is a date object that s related to the Helsinki timezone  Your options at this point are to use some 3rd party library  I recommend this   or hack-up the date object so you can use most of it s methods   Option 1 - a 3rd party like moment-timezone  moment 1270544790922  tz  Europe Helsinki   format  YYYY-MM-DD HH mm ss      outputs  gt  2010-04-06 12 06 30 moment 1270544790922  tz  Europe Helsinki   hour      outputs  gt  12   This looks a lot more elegant than what we re about to do next   Option 2 - Hack up the date object  var currentHelsinkiHoursOffset   2     sometimes it is 3 var date   new Date 1270544790922   var helsenkiOffset   currentHelsinkiHoursOffset 60 60000  var userOffset    date getTimezoneOffset   60000      min 60000   ms  var helsenkiTime   new Date date getTime    helsenkiOffset   userOffset      Outputs  gt  Tue Apr 06 2010 12 06 30 GMT-0700  PDT    It still thinks it s GMT-0700  PDT   but if you don t stare too hard you may be able to mistake that for a date object that s useful for your purposes   I conveniently skipped a part  You need to be able to define currentHelsinkiOffset  If you can use date getTimezoneOffset   on the server side  or just use some if statements to describe when the time zone changes will occur  that should solve your problem   Conclusion - I think especially for this purpose you should use a date library like moment-timezone

User · Answer

My solutions is to determine timezone adjustment the browser applies  and reverse it  var timestamp   1600913205     retrieved from unix  that is why it is in seconds    uncomment below line if you want to apply Pacific timezone   timestamp    -25200     determine the timezone offset the browser applies to Date   var offset    new Date    getTimezoneOffset     60        re-initialize the Date function to reverse the timezone adjustment var date   new Date  timestamp   offset    1000      here continue using date functions   This point the date will be timezone free and always UTC  You can apply your own offset to timestamp to produce any timezone

[javascript] How to ignore user's time zone and force Date() use specific time zone

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